The present invention provides a copolymer containing 80 mol % or more of a methyl (metl)acrylate unit (A), wherein the Vicat softening temperature thereof is 115° C. or higher, the saturated water absorption rate thereof at 60° C. is 4 % by weight or less, and the yellow index thereof is 1 or less. The process for producing the copolymer includes suspension polymerizing a monomer mixture containing 80 mol % or more of methyl (meth)acrylate (a) and 0.7 mol % to 7 mol % of (meth)acrylic acid (b), followed by melting and kneading the resulting precursor with an extruder to obtain a copolymer containing 80 mol % or more of a methyl (meth)acrylate unit (A), 0.45 mol % to 7 mol % of a (meth)acrylic acid unit (B), and 0.001 mol % to 0.25 mol % of a glutaric anhydride unit (C) wherein a conversion rate to the glutaric anhydride unit (C) is 0.1 % to 5 %.